1. Field of the Invention
The present invention relates to a process for the production of an aromatic polycarboxylic acid. More particularly, it relates to a process for reducing the optical density of terephthalic acid product comprising a liquid aqueous solution substantially saturated with terephthalic acid and comprising about 500 parts per million (p.p.m.) to about 6,000 p.p.m. of p-toluic acid at a temperature of about 204.degree. C. (400.degree. F.) to about 288.degree. C. (550.degree. F.).
2. Description of the Prior Art
Commercial crude terephthalic acid contains on a weight basis from about 800 p.p.m. to about 7,000 p.p.m. 4-carboxybenzaldehyde and about 200 p.p.m. to about 1,500 p.p.m. p-toluic acid as the main impurities and some crude terephthalic acid also contains lesser amounts, e.g., about 20 to about 200 p.p.m. of yellow-colored aromatic compounds having the structures of benzil, fluorenone or anthraquinone, which are characteristically yellow compounds as impurities resulting from coupling side reactions occurring during the oxidation of p-xylene. These yellow-colored aromatic compounds absorb light in regions of the ultraviolet and visible spectra where terephthalic acid does not absorb significantly. These yellow compounds by virtue of their absorption increase the "optical density" of purified terephthalic acid solutions and, therefore, optical density is used as a measure of the impurities of purified terephthalic acid.
U.S. Pat. No. 3,584,039, issued to Delbert H. Meyer, teaches a feasible, commercially useful method for purification of such commercially available crude terephthalic acid products by treating liquid phase solutions thereof in water at temperatures of 200.degree. C. (392.degree. F.) to 374.degree. C. (705.degree. F.) with hydrogen in the presence of a solid hydrogenation catalyst (e.g., metallic palladium on carbon support) and crystallizing terephthalic acid from catalyst-free liquid phase solutions at temperatures in the range of 50.degree. C. (123.degree. F.) to 150.degree. C. (302.degree. F.). The catalytic hydrogen treatment converts 4-carboxybenzaldehyde to p-toluic acid and decolorizes the terephthalic acid.
British Patent No. 1,152,575 is directed to the development of the Meyer Patent method for its commercial application by providing improved modes of conduct for the entire process from the step of dissolving crude terephthalic acid through the step of crystallizing terephthalic acid from the hydrogen treated aqueous solution. With respect to said crystallization, said British patent teaches the use of solvent evaporation to effect the cooling necessary to precipitate crystalline terephthalic acid, but cautions that conduct of such evaporative cooling should avoid shock cooling of the solution as would occur by instantaneous flash evaporation of solvent because such shock cooling coprecipitates dissolved impurities which contaminate terephthalic acid product. To prevent the contaminating effect of such shock cooling, the British patent teaches that the evaporative cooling should be controlled by evaporation against equilibrium back pressure, for example, by throttling of steam vapor exhaust at the equilibrium pressure. This is in effect a controlled rate evaporative cooling.
U.S. Pat. No. 3,931,305 assumes that the catalytic hydrogen treatment decolorizes the terephthalic acid and that the crystallization train is a means for separating unreacted 4-carboxybenzaldehyde and its hydrogenation product p-toluic acid from the remaining terephthalic acid. The process disclosed in U.S. Pat. No. 3,931,305 has been improved by virtue of a reduction in the O.D. content in excess of 90%, usually about 92%. This has been accomplished via the addition of 100 gm. to 600 gm. of water per 1,000 gm. of terephthalic acid to one or more crystallization zones, each zone being operated at a successively lower temperature. The initial optical density value of 1.183 was changed to a final optical density value of 0.089.
Crystallization by controlled rate evaporative cooling is, according to the above British patent, applied to continuous crystallization conducted in three series-connected stages under the conditions described to effect in 3.4 hours a 150.degree. C. (302.degree. F.) temperature drop from 277.degree. C. (530.degree. F.) initial solution temperature to the third stage temperature of 109.degree. C. (228.degree. F.). This mode of conducting said crystallization which provided an average cooling rate of 1.48.degree. F. per minute was not only inordinately slow but, when applied to aqueous solutions of terephthalic acid of 2,400 p.p.m. p-toluic acid content, also provided a terephthalic acid product containing 1,200 p.p.m. p-toluic acid. Such product would not be acceptable for direct reaction with ethylene glycol for polyester fiber manufacture.
The use of flash solvent evaporation induced crystallization of terephthalic acid from aqueous solution also containing dissolved p-toluic acid in amounts of 500 p.p.m. to 6,000 p.p.m. based on terephthalic acid can, without proper conduct thereof, bring into play the p-toluic acid contamination phenomenon alluded to in the British patent and more generally described in the later United States patent. Such contamination phenomenon is somewhat anomalous because, in spite of the fact that there is retained more than enough solvent water to prevent saturation or supersaturation with respect to p-toluic acid, p-toluic acid nevertheless comes out of solution. Said later United States patent suggests that the contamination phenomenon is in some way dependent on the rate of crystallization and the final temperature of crystallization and product separation and not solely on p-toluic acid concentration in the solution.
U.S. Pat. No. 3,497,552, issued to George P. Olsen, is directed to continuous crystallization of terephthalic acid in the presence of more soluble contaminants wherein water is injected into a large recycle loop for each crystallizer in a series of crystallizers, with crystallizer cooling occurring in the external recycle loop. Crystallization of an organic compound solute from solution is conducted without evaporating or otherwise removing solvent and without shock cooling of the solution of the organic compound solute.
Now there has been developed a process for reducing the optical density of terephthalic acid product, wherein water is added directly to one or more crystallizers in a series of crystallizers.